Mu peed himself during sex, Maru told him not to be embarrassed but ended up getting jealous because Mu used to do that type of kinky stuff with other guys. Ended with them just loving on eachother because jealousy and embarrassment are all petty things lol
i think he peed himself teehe
The kinkiness in this case is “male squirting”, related to “Urolagnia”, which is sexual excitement at the sight or thought of urination during sex. Maru purposely stimulated Mu by rubbing Mu’s urethral opening (urinary meatus) to induce urination, which embarrassed the hell out of Mu, even though it’s implied he had already done it before.
Lol I don't think it was planned. Even he's like, "Wtf just happened?"
I'm positive that from the beginning Maru operated with intent and I base my conjecture on both the visual presented in the manga and the dialogue which are pretty straight forward. In the scene on page 10:
http://www.mangago.zone/read-manga/otona_keikenchi_dj_boku_ni_dake_ijiwaru/uu/otona_keikenchi_dj_boku_ni_dake_ijiwaru-chapter-1/10/
we can see Maru stimulating Mu's urethral orifice by rubbing with his thumb, we can also see illustrated the "motion cues' denoting said stimulation, and on page 11, he continues rubbing it with his thumb, and we actually get the written cue "rubbing", and that's when Mu gushes out. Then on the scene on page 15 while talking with Yumeji and Shinkai:
http://www.mangago.zone/read-manga/otona_keikenchi_dj_boku_ni_dake_ijiwaru/uu/otona_keikenchi_dj_boku_ni_dake_ijiwaru-chapter-1/15/
Maru explicitly states "I thought he would be into that kind of extreme play". Then, the clincher for me is on page 19, where Maru looks at Mu puzzled and tells him that he had thought for sure Mu had done that level of play before.
The idiomatic expression of "I thought" denotes an expectation or belief before an event has happened, meaning that Maru had already thought about it before engaging in the act. If on the other hand he had used the expression " I would have thought that you had done this before" then that would have denoted an expectation after the event has already happened, meaning that Maru only thought about it after the fact. There's no doubt he walked in there with kinky urolagnia intentions :D
I don't really get what happened teehee (≧∀≦)